The present invention relates to air data measurement systems. More particularly, the present invention relates to acoustic air data measurement systems.
Aircraft air data systems commonly utilize pitot tubes and/or multi-function probes to measure air data related pressures. These aircraft air data systems utilize the measured pressures to calculate air data parameters for an aircraft, such as airspeed and angle of attack. Other air data systems utilize acoustic sensors, instead of pressure sensors, in order to estimate parameters such as local airspeed and local angle of attack of the aircraft.
These acoustic air data systems can be based upon the principle that the transmission time of sound waves in a fluid along a given path is a function of the local acoustic velocity and the local fluid velocity component parallel to that path. Thus, some of these systems are based on the concept of transmitting an acoustic signal generated by an electromechanical transducer (e.g., a sound source) through the fluid medium to one or more downstream receivers, and measuring the travel time to each downstream receiver. Still other acoustic air data systems utilize only passive acoustic sensors, and do not utilize an electromechanical transducer or sound source. One such air data system is disclosed in U.S. Pat. No. 5,585,557 to Loschke et al., which issued on Dec. 17, 1996, and which is hereby incorporated by reference in its entirety.
In the air data system disclosed in the Loschke et al. patent, an equidistant array of microphones are set a certain distance from a reference microphone. The angle of attack of the airflow is measured by finding the microphone in the array of microphones whose output has the highest correlation coefficient with the reference microphone output. The corresponding assumption of attached flow is then made. Since this known configuration allows for flow measurement only in the direction from the reference microphone to one of the microphones in the array of microphones, this air data system suffers the disadvantage that the resolution for the angle of attack measurement is limited by the number of microphones in the equidistant array. In order to achieve high resolution in the angle of attack measurement or estimation, a large number of array microphones must be used. This increases the cost of the system. Using a large number of array microphones also increases the system complexity, making manufacturing and installation of the system more difficult, and potentially reducing the field reliability of the system. A second disadvantage of this type of system is that air data can only be inferred if the flow direction is from the reference microphone to the array. In other words, these systems are uni-directional. This limits the usefulness of these systems. For example, a uni-directional system has limitations in uses such as helicopters.
Consequently, an acoustic air data system which overcomes one or more of the previously described problems, or other problems not described, or which provides advantages over prior art air data systems, would be a significant improvement in the art.